Method and apparatus for reverse sputtering selected electrically exposed areas of a cathodically biased workpiece



Nov. 12, 1968 F. BARSON ETAL 3,410,774

METHOD AND APPARATUS FOR REVERSE SPUTTERING SELECTED ELECTRICALLY EXPOSED AREAS OF A CATHODICALLY-BIASED WORKPIECE Filed Oct. 23, 1965 2 Sheets-Sheet l FIG.1

a 5 26 MILVIIIlII'II/A. 27b 9 34- 28 33 23 [36 L 30w 1. f\ l l L 1 6 52 32 0 gOgVER 5 U PLY 9 '7! (9 (9 @G) (-9 42 27c \7 l 1 l I INERT GAS -14 VACUUM RESERVOIR PUMP INVENTORS FRED BARSON JOHANN STURM AGENT Nov. 12, 1968 F. BARSON ETAL METHOD AND APPARATUS FOR REVERSE SPUTTERING SELECTED ELECTRICALLY EXPOSED AREAS OF A CATHODICALLY BIASED WORKPIECE Filed Oct. 23, 1965 2 Sheets-Sheet 2 FEG. 2

United States Patent METHOD AND APPARATUS FOR REVERSE SPUTTERING SELECTED ELECTRICALLY EXPOSED AREAS OF A CATHODICALLY BIASED WORKPIECE Fred Barson, Wappingers Falls, and Johann Sturm, Poughkeepsie, N.Y., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed Oct. 23, 1965, Ser. No. 502,986 4 Claims. (Cl. 204-192) ABSTRACT OF THE DISCLOSURE A reverse sputtering method and apparatus for removing surface contaminants from selected areas of a workpiece where the selected areas are exposed through an insulating layer. The workpiece is biased to function as the cathode [and a cathodically biased mask, having apertures conforming to the shape of each selected area but larger in size, is axially aligned and positioned from the workpiece a distance which will not permit a DC ionic charge to occur.

This invention relates to a process and apparatus for removing surface contaminants from materials; and more particularly to a process and apparatus for removin oxide and stains from selected surface areas of semiconductor substrates by ion bombardment, also commonly referred to as cathodic sputtering.

The phenomenon of cathodic sputtering refers to the dislocation of atoms or molecules from the surface of a material by the impact energy of gas ions which are accelerated in an electric field. Cathodic sputtering is established by the creation of a glow discharge between an anode and a cathode, wherein the current therebetween is composed of electron flow to the anode and positive ion flow to the cathode. The ions are created by ionization of gas molecules existing within the glow discharge region between the anode and cathode. The ionization results from collisions of the gas particles with the electron flow from the cathode to the anode.

The removal of surface contaminants by cathodic sputtering is known in the art and may be referred to as reverse sputtering since it is opposite to the process of cathodic sputtering wherein substances are deposited onto the surface of a material. For example, reverse sputtering has been observed in electric-arc-inert-gas welding applications wherein contaminants are removed from the surfaces of the materials to be welded prior to the actual weldment.

Reverse sputtering has been used to clean large surface areas of a semiconductor as a preliminary step in the manufacture of semiconductor devices such as photoelectric cells. In such applications, mask or holder assemblies, having suitably placed openings, hold the material in position in the ion stream and prevent ion bombardment of surface areas from which contaminants are not to be removed. However, the use of prior art masking techniques is unsatisfactory where contaminants are to be removed from extremely small selected surfaces areas (in order of 6-10 mil diameter). A uniform cleaning of the selected surface area is not obtained as the ion beam tends to concentrate in the center of the selected area with little, if any, sputtering of the peripheral areas. This also produces etching of the surface material which leaves undesirable pin-holes in the surface. These effects are attributable to the accumulation of a positive ion charge on the surface of the ma king material which focuses the ion beam in the centers of the selected areas.

3,410,774 Patented Nov. 12, 1968 "Ice The resulting high electrostatic fields from such charge accumulations may often exceed the dielectric breakdown strength of the masking material resulting in destructive arcing to the surface of the contaminated material.

Furthermore, residual contaminants left on such selective surface areas as a result of non-uniform sputtering decreases the reliability of semiconductor devices. For example, the subsequent deposition of substances over unclean surface areas increases the probability of poor bonding between the deposition and the semiconductive surface. Such poor bonding also creates undesirable electrical resistance. The presence of such deleterious mechanical .and electrical interfaces increases reliability hazards resulting in increased operational failures of the finished semiconductor product.

Because of such difliculties of the prior art masking techniques, attempts have been made to apply known chemical and/or physical abrasive cleaning techniques such as chemical etchants or abrasive microcloth to remove contaminants from extremely small and confined surface areas. The use of such methods, however, does not achieve a uniform cleaning of the surface area primarily because of its restricted size and confinement. In addition, and especially with chemical etching techniques, the cleaning agent attacks those areas adjacent to the selected areas, resulting in damage to the workpiece. The adjacent area may be attacked to the extent that undercutting results. Also, in many instances the surface contaminant or stain itself is resistant to the chemical of abrasive agent.

It is accordingly an object of this invention to provide an improved reverse sputtering process and apparatus for the uniform and controlled removal of surface contaminants from small selected areas of a material.

An additional object is to prevent the accumulation of charge on the mask and resulting arcing from the mask to the surface material.

It is a further object of this invention to apply reverse sputtering to remove contaminants from small, confined surface areas resistant to known chemical and/or abrasive cleaning agents.

A further, additional object is to remove contaminants from small selected surface areas of a material wherein the removal of material from adjacent areas can be uniformly controlled or eliminated.

In accordance with one aspect of the invention, the above objects are achieved in a reverse sputtering process by interposing a perforated, electrically conductive mask in the ion stream adjacent to the cathode to expose only selected surface areas of the cathode from which surface contaminants are to be removed. To provide uniform sputtering of the selected surface areas, the potential of the mask is maintained at the cathode potential by electrically interconnecting the mask and cathode. The thickness of the mask is selected to be a given ratio of the largest dimension (diameter, length of longest segment) of the surface area. The mask is selected from a material having high resistance to sputtering.

In another aspect of the invention, the mask is made an integral part of the cathode by forming a layer on the surface of the cathode using known deposition or evaporation techniques and then etching the layer prior In the drawings:

FIG. 1 is a diagrammatic representation of a simple form of apparatus for practicing the invention illustrating the environment in which the process is carried out.

FIGS. 2a, b are enlarged detail views showing preferred embodiments of the mask and its interrelationship with a contaminated surface.

The description and operation of the reverse sputtering apparatus and process will be described with reference to the figures wherein like reference numerals are used throughout to designate like elements.

Description Reverse sputtering requires a controlled inert atmosphere to provide the necessary gas ions for the sputtering action and to prevent the recontamination of the materials surface. Referring to FIG. 1, tightly sealed vacuum chamber 2 isolates inert-gas atmosphere 19 such that the necessary environmental condition can be created to establish reverse sputtering. Base 4 of vacuum chamber 2 has vacuum port 12 through which controlled amounts of the atmosphere 19 can be removed to reduce the pressure within vacuum chamber 2. Inert-gas is introduced into vacuum chamber 2 through inert-gas port 18 in base 4.

The pressure within vacuum chamber 2 is reduced by vacuum pump 8 through conduit 11 which is connected to vacuum port 12 in base 4. The vacuum pressure is measured and controlled by vacuum pressure valve and meter 10. Inert-gas enters vacuum chamber 2 through inert-gas port 18 via gas conduit 17 which connects with inert-gas reservoir 14. The amount of inert-gas admitted to vacuum chamber 2 is controlled by inert-gas valve and meter 16, which may be any type of well known micrometer needle valve, such as a Whitney micrometer needle valve. Regardless of the type of valve used, it must be capable of measuring pressure in microns of mercury.

High voltage DC power supply 6 provides the necessary electrical power to initiate and sustain an arc discharge between cathode-mask assembly 33 and anode 44. Cathode lead connects the negative terminal of power supply 6 to the cathode or surface contaminated substance 22. Cathode lead 5 enters vacuum chamber 2 through high voltage feed-through 27a and then is passed within hollow holder assembly 24 and attached to surface contaminated substance 22. For the purposes of the present invention, lead 9 is connected from the negative terminal of power supply 6 to a conductive wafer mask 30 through high voltage feed-through 27b and hollow mask holder assembly 36.Anode lead 7 connects the positive terminal of power supply 6 to anode 44 through high voltage feedthrough 27c and hollow anode holder assembly 42. For optimum results, it is preferred that the power supply 6 have the capability of providing between 1300 and 1500 volts at a current of 3 to milliamps per square inch of cathode for a sustained period of to minutes.

Cathode-mask assembly 33 comprises surface contaminated substance 22, insulating substrate 26 having holes 28 and conductive wafer mask 30 with apertures 32. Con: ductivewafer mask 30 is positioned with respect to surface contaminated substance 22 and insulating substrate 26 such that apertures 32 are substantially in alignment with holes 28. Mask holder assembly 36 retains the above alignment of conductor wafer mask 30 as well as holding conductor wafer mask 30 in spaced relationship with insulating substrate 26 and surface contaminated substance 22.

The preferred spacing between anode 44 and cathodemask assembly 33 is approximately /2 inch for the conditions of inert-gas pressure and voltage specified herein. Those skilled in the art will recognize thatthe anode-tocathode spacing can be decreased such that the anode is positioned just outside the well-known dark space which exists in the region between the anode and cathode in any cathodic sputtering process. The anode can be extended as far away from the surface contaminated substance 22 to a distance wherein the arc is not extinguished.

Ill)

Operation To establish the necessary atmosphere 19 within the vacuum chamber 2, vacuum pump 8 decreases the pressure within vacuum chamber 2 through port 12 and conduit 11 to approximately 5x 10- millimeters of mercury which is measured by vacuum pressure valve and meter 10. A controlled amount of inert-gas, such as argon, is admitted to vacuum chamber 2 through port 18 and conduit 17 from inert-gas reservoir 14. Approximately 5075 microns of argon is admitted to vacuum chamber 2 to provide a sufiicient atmosphere 19 for the reverse sputtering process. The inert-gas flow into vacuum chamber 2 is controlled by inert-gas valve and meter 16. It will be recognized by those skilled in the art that any inert-gas can be used to form atmosphere 19, but argon is preferred as it is a heavy gas and the relatively large mass of its ions compared to other inert-gases provides greater reverse sputtering.

With the above controlled atmosphere 19 created within vacuum chamber 2 and the positioning of the cathodemask assembly 33 and anode 44, described supra, power supply 6 is activated and an arc is struck between surface contaminated substance 22 and anode 44. The establishment of the are generates argon ions in the space between the cathode assembly 33 and anode 44. The positive argon ions 50 are accelerated by the potential between the anode 44 and the cathode-mask assembly 33 towards the surface contaminated substance 22. Without the conductive wafer mask 30, which is an important aspect of the invention, the ions would accumulate on the surface of insulating substrate 26 and this accumulated positive charge surrounding holes 28 causes the ion stream 50 to be focused in the center of selected surface areas 38. The focusing of the ion stream 50 in the center of the selected surface areas 38 causes holes to be drilled in the surface contaminated substance. The ion beam does not bombard the peripheral portions of selected surface area 38 and consequently it is not uniformly cleaned.

However, with the use of the conductive wafer mask 30, which is maintained at the same potential as the surface contaminated substance 22, the aforementioned charge accumulation of argon ions 50 on insulating substrate 26 is prevented. This happens because the apertures 32 are aligned with the holes 28 such that select surface areas 38 are exposed to the argon atoms 50. The potential on conductive wafer 30 causes the argon ions 50 to fan out such that select surface areas 38 of surface contaminated substance 22 are uniformly bombarded without the resulting aforementioned pinhole effect. In addition, because the ion charge accumulation on insulating substrate 26 is prevented, the possibility of dielectric breakdown of insulating substrate 26 is eliminated.

'A more detailed view of the cathode-mask assembly is shown in FIG. 2a. In this embodiment silicon semiconductor substrate 39 comprises silicon wafer 20,- a substrate of silicon oxide 21 formed on top of silicon wafer 20, and a molybdenum land 23, which partially extends over silicon oxide substrate 21 and is enclosed by high temperature glass layer 26. The process for manufacturing the silicon semiconductor substrate 39 (not a part of the invention) requires that the molybdenum substrate 23 be oxidized'prior to applying the glass layer 26 to remove residues formed during prior process steps in its manufacture. A hole 28 has been formed in glass layer 26 with a known chemical etching process using a chemical etchant such'as a mixture of nitric and hydrofluoric acids (HNO -HF) or a fluorboric acid and hydrofluoric acid solution (HBF -HF). The etching of hole 28 leaves a layer of molybdenum oxide 40 upon the surface of the molybdenum land 23 at the bottom of hole 28. Prior to contacting the molybdenum land 23, it is desirable to remove molybdenum oxide layer 40 from the selected surface area 38 of the silicon semiconductor substrate 39.

To effect the removal of oxide layer 40, conductive wafer mask 30, having apertures 32, is placed in spaced relation to the silicon semiconductor substrate 39 as described supra. It is to be understood that FIG. 2a is only a diagrammatical representation of semiconductor substrate 39 and the conductive wafer mask 30. Semiconductor substrate 39 may have a plurality of holes 28 exposing a plurality of selected surface areas 38. In such an instance, it is to be understood, that conductive wafer mask 30 would have a corresponding number of apertures 32 which would be aligned with holes 28 to expose selected surface areas 38 to the ion stream 50. In the embodiment of FIG. 2a the diameter 29 of hole 28 is 6 mils. The diameter 31 of aperture 32 is approximately 1 to 2 mils larger than diameter 29 of hole 28, as it is desirable to remove some surrounding glass from glass layer 26 in the process of removing the molybdenum oxide layer 40 from the selected surface area 38. The spacing 34 of the conductive wafer mask 30 with respect to the top surface of insulating glass layer 26 is a maximum of 1 to 2 mils. However, as will be described hereinafter, conductive wafer mask may be in actual contact with the top surface 25 of insulating glass layer 26.

Conductive wafer mask 30 should be fabricated from a substance which is highly resistive to reverse sputtering. Aluminum, molybdenum and chromium appear to possess the desired characteristics of electrical conductivity and high resistance to the bombardment of argon ions or other inert-gas ions. These metals can be made more resistive to reverse sputtering by forming a layer of oxide on their upper-most surface, as the oxides of these metals are more resistive to ion bombardment than the metals themselves. It is preferred that the thickness 35 of the conductive wafer mask 30 be no greater than /2 the diameter of aperture 31.

Uniform removal of the molybdenum oxide layer from selected surface area 38 is achieved by preventing the formation of an ion charge on the top surface 25 of insulating glass layer 26 by maintaining the potential of conductor wafer mask 30 at the same potential as the silicon substrate 20. As shown in FIG. 2a, conductive lead 9 provides the electrical connection between the conductive wafer mask 30 and the silicon substrate 20.

FIG. 2b shows another aspect of the inventive apparatus wherein conductive wafer mask 30 is shown as a deposited film on the top surface 25 of insulating glass layer 26. In such an application, the conductive Wafer mask 30 can be formed by known deposition techniques on top surface 25 of insulating glass layer 26; and hole 29 and aperture 31 etched in insulating glass layer 26 and conductive wafer mask 30 by known chemical etching techniques as mentioned supra. In addition to the requirements of electrical conductivity and high resistance to reverse sputtering, the conductive wafer mask material must also possess essentially the same thermal expansion characteristics as the silicon wafer. Molybdenum appears to have the three aforementioned characteristics and forms a satisfactory conductive wafer mask when deposited in thicknessess equal to approximately 5000 Angstroms. The potental of the deposited conductive wafer mask 30 is maintained at the same potential as the silicon layer 20 by interconnecting the two via lead 9.

The conductive wafer mask 30 is subject to attack from the ion stream, however, as its thickness exceeds that of the molybdenum oxide layer 40, it will not be sputtered away prior to the removal of the molybdenum oxide layer 40. Consequently, the attrition of the deposited conductive wafer mask 30 is not a problem. However, in the application disclosed in FIG. 2a wherein a separate conductive wafer mask 30 is used, the continuous attrition of the conductive wafer mask 30 requires that it be replaced periodically. Those skilled in the art will recognize that the cleaning process is not limited to the removal of molybdenum oxide. For example, the oxides of other metals, such as aluminum, can also be removed.

While the invention has been described and illustrated in the form of a few particular embodiments, it will be understood by those skilled in the art that variations and modifications may be made without departing from the scope of the invention as claimed.

What is claimed is:

1. A reverse sputtering process for removing surface contaminants from a plurality of selected surface areas of a surface contaminated material upon a semiconductor body, said surface contaminated material comprising part of a cathode-mask assembly, including the steps of:

placing an insulating substrate upon said surface contaminated material;

exposing said plurality of said selected surface areas through said insulating substrate;

positioning an electrically conductive wafer mask with a like plurality of apertures as said selected surface areas at a distance from said insulating substrate within which the formation of a DC ionic charge on said wafer mask and said insulating substrate will not occur when said wafer mask is energized, each individual one of said plurality of apertures aligned with an individual one of said plurality of selected surface areas, said aperture being of a size greater than the size of said selected areas so as to allow fan-out of the impinging ions so as to impinge at least upon the entire area of each of said selected areas;

aligning said cathode-mask assembly comprised of said surface contaminated material, said insulating substrate, and said electrically conductive wafer mask, to the reverse sputtering anode to allow the ion beam to impinge upon said selected areas; and

energizing said electrically conductive wafer mask and said surface contaminated material to the same DC potential during sputtering, said energizing preventing the formation of a DC ionic charge on said electrically conductive wafer mask and said insulating substrate, whereby uniform sputtering of said plurality of said selected surface areas is obtained.

2. In a reverse sputtering apparatus for removing surface contaminants from a plurality of selected surface areas of a surface contaminated material by exposure of said selected surface areas to a bombarding ion beam of said apparatus, wherein said apparatus comprises an anode, a cathode-mask assembly, a power supply, and a controlled atmosphere system; the cathode-mask assembly comprising:

an insulating substrate upon the surface of said surface contaminated material, said insulating substrate having a plurality of openings therein exposing said selected surface areas for contaminant removal therefrom, said openings being of the same size as said selected areas; and

an electrically conductive wafer mask having a plurality of apertures, there being one aperture for each selected surface area, the apentures being greater in size than said selected areas to allow fan-out of the bombarding ions so as to impinge on at least the entire area of each of said selected areas, said wafer mask positioned at a distance from said insulating layer within which the formation of a DC ionic charge on said insulating layer and said wafer mask will not occur when said wafer mask is energized, the apertures and said selected surface areas being aligned to expose said selected surface areas to the ion beam, said electrically conductive Wafer mask bein maintained at the same DC electrical potential as said surface contaminated material, whereby a uniform sputtering of said selected surface areas is obtained.

3. The reverse sputtering apparatus as in claim 2 wherein said electrically conductive wafer mask has a thickness no greater than one-half the diameter of said selected surface areas exposed through said insulating substrate.

4. The reverse sputtering apparatus as set forth in 7 8 claim 2wherein said electrically conductive wafer mask 3,341,442 9/1967 Kay 204192 is positioned at a distance not greater than .002" from 3,361,659 1/1968 Bertelsen 204192 said insulatlng substrate. I OTHER REFERENCES R f es Cited 5 G. V. Spivak et 21., Doklady Akademii Nauk, SSSR, vol. 104, pp. 579-581, 1955, ibid., vol. 114, 1957, No. 5, UNITED STATES PATENTS pp. 1001-3.

2,702,274 2/ 1955 Law 204-192 ROBERT K. MIHALEK, Primary Examiner. 

